Catalyst structures are employed to promote a variety of high-temperature processes involving reactions such as the partial oxidation of hydrocarbons, the complete oxidation of hydrocarbons for emissions control and efficiency, reactions in catalytic mufflers for automotive emissions control and the catalytic combustion of fuels for further use in gas turbines, furnaces and the like. Generally, catalytic combustion involves mixing fuel and air and passing this mixture through a catalyst structure to effect a combustion reaction. As a result of the combustion process, very high gas temperatures are generated. These high gas temperatures, although favorable for turbine efficiency, subject the catalyst structure to thermal stresses. In addition to thermal stresses, the catalyst structure is also subject to a very high axial force in the direction of gas flow. This axial force arises from the resistance to gas flow created by longitudinally disposed channels of the catalyst structure. Some catalyst structures do not have the intrinsic strength to withstand this axial load and must rely on a catalyst support structure typically located downstream of the catalyst. The support structure is likewise subject to the heavy thermal and mechanical loads that the catalyst structure suffers and must be designed to account for these and other important performance considerations.
Referring now to FIGS. 1 and 2, a typical catalytic combustion reactor 1 is shown in FIG. 1. As shown, a catalyst structure 2 is positioned in a generally cylindrical combustion reactor 1 downstream of a preburner 3 and generally perpendicular to the flow 4 of an oxygen-containing gas. Typically, this gas is an air and fuel mixture, the fuel being introduced to the monolithic catalyst structure 2 via fuel injector 5 and the high velocity air 11 being introduced via a compressor (not shown). The catalyst structure 2 is positioned in this manner to obtain a uniform flow of air/fuel mixture through the catalyst, and to allow the mixture to pass through passageways that extend longitudinally through the catalyst structure 2. In order to maintain the catalyst structure 2 in a stable position in the combustion reactor 1, it is necessary to employ some type of support means or structure to secure the catalyst structure to the combustion reactor, including, as one possibility, a support structure 6 which abuts the outlet side 7 of the catalyst structure 2 to support the axial load on the catalyst. As used herein, the “outlet side” 7 of the catalyst structure 2 is the side where the partially or completely combusted air/fuel mixture exits the catalyst structure 2. Therefore, the “inlet side” 8 of the catalyst structure 2 is the side where the uncombusted air/fuel mixture is initially introduced to the catalyst structure 2. The support structure 6 preferably has a very open structure so that it provides minimal inhibition of gas flow. As shown in FIG. 2, the support structure 6 transfers this axial load to a cylindrical structure 9 via a ledge 10 mounted on the inside of the cylindrical wall or lines 9. Examples of several supporting systems are described in U.S. Pat. No. 5,461,864 to Dalla Betta et al., U.S. Pat. No. 6,116,014 to Dalla Betta et al., and U.S. Pat. No. 6,217,832 to Dalla Betta et al. all incorporated in their entirety herein by reference. The high velocity gas flow 4 in the combustor cylinder 9 generates a significant pressure drop across the catalyst structure 2 and, hence, load upon the catalyst structure 2. It is this load that the support structure 6 must be able to withstand. To understand how this pressure drop is generated, a typical catalyst construction will now be discussed.
A typical catalyst structure 2 can be a corrugated, wound arrangement made up of a multitude of longitudinally disposed channels for the passage of the combustion gas mixture. At least a portion of the channels is coated on their internal surfaces with a combustion catalyst. Examples of typical catalyst structures are found in U.S. Pat. No. 5,250,489 to Dalla Betta et al., U.S. Pat. No. 5,511,972 to Dalla Betta et al., U.S. Pat. No. 5,183,401 to Dalla Betta et al., and U.S. Pat. No. 5,512,250 to Dalla Betta et al., all incorporated in their entirety herein by reference. Generally, corrugated metal foil is coated with a catalyst layer and then spiral wound into a cylindrical structure. Such a catalyst unit has longitudinal channels for gas flow. As gas passes through the unit at high flow rate, the resistance to gas flow through the channels results in an axial load on the catalyst structure 2 that attempts to move the foil in the direction of flow. If the catalyst structure 2 is attached to the combustor at the outer circumference, and if the axial force exceeds the foil to foil sliding frictional resistance in the wound structure, then this axial force will cause the catalyst foils to telescope in the direction of gas flow. The pressure drop across the catalyst structure 2 is typically in the range of 1 to 5 pounds per square inch (psi). For a catalyst system with a diameter of 15 inches, for example, this would result in a force on the catalyst of 180 lbs. at a pressure drop of 1 psi and a force of 900 lbs. at a pressure drop of 5 psi. If a multistage monolithic catalyst structure 2, for example, such as that described in U.S. Pat. No. 5,183,401 to Dalla Betta et al., is employed as a 20-inch diameter catalyst in a catalytic combustion reactor where the air/fuel mixture flow rate is about 50 lbs./second at a pressure drop through the catalyst of 4 psi, the total axial load on the catalyst would be about 1,260 lbs. In essence, the support structure 6 must be able to support a catalyst structure 2 undergoing significant axial forces.
Not only are the axial forces upon the support structure significant, but also, the temperatures within parts of the combustor are very high relative to high performance material strength. The temperature of the catalyst structure can change rapidly while in use and temperatures approaching and even exceeding 1,000° C. are possible. As a result, thermal gradients are quite common in catalytic combustion and a support structure that is designed to withstand a nonuniform temperature is important. A typical operating transient is shown in FIG. 3 where a typical gas turbine system is started up using the combustor system described in FIGS. 1 and 2. FIG. 3 shows the temperature of several components during a start transient. The turbine is started at time 12 by igniting the preburner 3 of combustor in FIG. 1. The average temperature of the gas flowing through the support structure 6 is shown as line 14. The temperature of the cylindrical combustor liner 9 is shown as line 16. As can be seen in FIG. 3, the high temperatures cause the relatively thin-walled, uncooled support structure 6 to thermally expand by a significantly greater magnitude than the relatively thick-walled reaction chamber wall 9 that has a cooler air flowing on one side. As a result, thermal expansion differences between components are generated. To overcome this problem and avoid cracking or deformation of the catalyst structure 2 and support structure 6, the support structure 6 and catalyst structure 2 are generally sized so that their outside diameters are smaller than the inside diameter of the reaction chamber wall 9 to allow thermal expansion of the catalyst structure 2 and support structure 6 during such high temperature operation. If the outside diameter of the support structure is too large, the support structure 6 is unable to thermally expand resulting in possible damage to the support structure 6 itself and to the foils of the catalyst structure 2. Not only are the expansion differences between components problematic, but also, the combination of the large axial loads and high temperatures cause significant deformation of the support structure 6.
For example, FIG. 4 illustrates a sectional view of a catalyst support structure 18 having a monolithic open celled or honeycomb-like structure as described in detail in U.S. Pat. No. 6,116,014 to Della Betta et al. The support structure 18 is formed by thin strips 20 of high temperature resistant metal or ceramic which abut against the outlet side of the catalyst structure 2, and extend in a direction perpendicular to the longitudinal axis of the catalyst structure to essentially cover an outlet side of the catalyst structure 2. The strips 20 making up the support structure 18 are bonded together to form a bonded metal monolith where the contacting flat portions 22 of the strips 20 are joined together by welding or brazing. The bonded metal monolith when exposed to rapidly changing temperature and thermal gradients generates high thermal stresses within the honeycomb structure. Furthermore, the contacting flat portions 22 inhibit independent expansion and contraction of individual strips in response to localized thermal gradients. As a result, stress concentrations at the contacting flat portions 22 may lead to failure of the bonds and cause fatigue, cracking and deformation. Gross failure may lead to failure of the part, a short useful life, and the possible dislocation of a portion of the individual strips 20 resulting in a free body in the system that may threaten turbine integrity downstream. Minimizing the number of joined, redundant structural members increases the freedom of individual axial supports or struts to expand and contract in response to localized thermal-mechanical stresses without imposing stresses on neighboring axial supports or struts. The minimizing of joined, redundant structural members alone or in combination with a construction that allows individual axial supports to expand and contract freely is an important design consideration that has not been addressed by previous inventions. The present invention provides a support structure arrangement having axial supports or struts that are free to expand and contract in response to thermal stresses.
A related design consideration is the facility to which the design lends itself to scalability. To use the honeycomb-like structure discussed above, for example, a support structure having a larger diameter would require a factor of additional welds. A smaller support structure having smaller channels would make welding more cumbersome. This reality associated with either an increase or decrease in size would naturally decrease the ease of manufacture and increase the cost of the support structure. As always, a design that does not substantially increase the cost, time, or difficulty of manufacture with respect to scale is desirable. The present invention sets forth such a support structure design.
Furthermore, a catalyst support structure should minimally obstruct airflow while simultaneously providing uniform support. If struts of the support structure are rather widely spaced over the face of the catalyst, then high local contact forces or stresses will result. In certain portions, these contact forces can exceed the strength of the thin catalyst foil resulting in deformation of the foil under high loads. One solution to this foil deformation problem is to provide more supporting axial supports in order to reduce the contact stress with the catalyst foils at the outlet face of the catalyst. However, an increased number of axial supports will increase the blockage of gas flow and increase the overall pressure drop in the combustor system. In the honeycomb-like design, the support-to-support distance varies widely. For example, at weld locations 22 the strips 22 abut each other and, in effect, provide non-uniform support relative to non-weld locations. Also, the blockage of gas flow is increased at weld locations 22 where there is at least a doubling of strips. This doubling of thickness does not result in uniform support and tends to reduce the efficiency of the gas turbine by decreasing airflow.
Thus, it is desirable to design a support structure that provides the least restriction of air flow through the catalyst, uniform support to the catalyst foils, fewer stress concentrations, and members that are free to expand and contract in response to localized thermal gradients. The present invention is directed at satisfying the aforementioned and additional needs in catalyst support structure construction and design.